The present invention relates to sensing the magnitudes of low level primary currents in a current transformer, and particularly to sensing the magnitude of differential primary current in a differential current transformer as utilized in personnel ground fault protective devices.
To afford adequate personnel protection against the hazards of ground leakage current, ground fault protective devices must respond to differentials in the currents flowing in the line and neutral sides of a protected circuit of 5 milliamperes or more. Traditionally, a differential current transformer is utilized to sense when the 5 milliamp threshold is exceeded. Obviously, 5 milliamp net primary excitation is exceedingly small, and yet sufficient secondary signal must be developed for reliable processing by electronic circuitry pursuant to initiating a ground fault trip function. To achieve this, the differential transformer core must utilize relatively expensive materials, particularly in view of the compact sizes required for implementation in existing ground fault circuit interrupting (GFCI) devices, and the electronic signal processing circuitry must be of a sophisticated design having high signal gain capabilities and low susceptibility to noise. Furthermore, sensitivity to the typical 5 milliamp differential current trip threshold is highly dependent on the core characteristics, which, in turn, vary with temperature.
It is accordingly an object of the present invention to provide improved apparatus for accurately sensing current transformer primary current or differential primary current.
An additional object is to provide apparatus of the above character wherein the sensation of primary current is relatively unaffected by current transformer core characteristics.
Yet another object is to provide apparatus of the above character wherein the sensation of primary current is relatively unaffected by variations in core characteristics.
A further object is to provide apparatus of the above character which is capable of producing relatively large signal outputs indicative of primary current.
Other objects of the invention will in part be obvious and in part appear hereinafter.